1. Field of the Invention
This invention relates to metalworking fluids, which are employed during metal machining operations such as drilling, cutting, turning and grinding. The fluid serves to lubricate and cool the tool and workpiece. Heat and friction which can cause severe tool wear and distortion of the workpiece are minimized by the fluid. The fluid also serves to flush chips and particles of metal from the machined area. Typically, these functions are carried out during the machining operation by directing the metalworking fluid onto the surface being machined, whereafter the fluid flows into a reservoir or sump and is recycled by a pump to again impinge onto the metal surface. Metalworking fluids can be nonaqueous oils, oil-water emulsions, and aqueous solutions. The latter two are considered water-based metalworking fluids for the purposes of the present invention.
The conditions under which metal machining operations are carried out favor corrosion of the machined metal surfaces, tooling and other machine elements during and following the machining operation. The risk of corrosion is particularly severe when the metalworking fluid is water-based, that is, an aqueous solution or an oil-water emulsion. For this reason, the water-based metalworking fluids will usually contain small amounts of one or more corrosion inhibitors. In addition to the lubricating, cooling and corrosion-inhibiting functions, the metalworking fluid should possess to as great a degree as possible the following important qualities: it should be usable with all metals it might contact, both ferrous and nonferrous, and it should preferably protect all such metals against corrosion; the fluid should be non-foaming, and any foam formed during use should decay rapidly; the fluid should not be opaque, and it should preferably be transparent so that the machine operator can easily view the work area; the fluid should be useful when subjected to dilution with water of high hardness as well as low hardness content; it should not cause dermatitis or present any other hazards to the health of the machine operator; and preferably it should be disposable by discharge into a sewer.
A widely used corrosion inhibitor for water-based metalworking fluids is a mixture of an alkanolamine such as triethanolamine ("TEA") and a nitrite compound, typically an alkali nitrite such as sodium nitrite. Typically, the TEA is present in the fluid in amounts of about 0.02 to about 0.8 wt. % together with sodium nitrite in amounts from about 0.8 to about 0.02 wt. %, such that the ratio of TEA to sodium nitrite is about 1:40 to about 40:1 by weight.
There is interest in decreasing the necessary concentrations of TEA and nitrites in metalworking fluids while maintaining or improving corrosion inhibition, and there is a need for a corrosion inhibitor to replace the TEA-nitrite corrosion-inhibition system in water-based metalworking fluids. Such a corrosion inhibitor should be readily available, relatively inexpensive and should not pose health or safety hazards.
Some substances have been introduced as alternative corrosion inhibitors to the TEA-nitrite corrosion inhibition system but their corrosion inhibiting effectiveness is generally acknowledged to be less than that of the TEA-nitrite system and they are usually required to be used at higher concentrations for comparable protection of ferrous metals. In addition, some of these alternative corrosion inhibitors do not protect nonferrous metals from corrosion.
2. Description of the Prior Art
The molybdate anion (MoO.sub.4.sup.-2) has been described to inhibit corrosion in certain applications but the prior art has not recognized or suggested that particular molybdate-based corrosion-inhibiting fluids of the present invention. For instance, molybdates have been disclosed to inhibit corrosion in alcohol-based antifreeze solutions in U.S. Pat. No. 2,147,409, which discloses the utility of 0.05 to 1% of molybdate, and in U.S. Pat. No. 2,147,395, which discloses the utility of 0.05 to 1% of molybdate and 0.05 to 1% of nitrite. Molybdates have also been described as inhibiting corrosion of ore grinding mill liners and balls, and molybdates have been added as corrosion inhibitors to cooling tower waters, hydraulic fluids, and paints. These disclosures do not recognize the particular aspects of the present invention in water-based metalworking fluids.
West German published patent application No. 2,351,274 discloses aqeuous metalworking fluids containing a metallic salt of an anion selected from a list of some 24 different anions. The list includes molybdates and such anions as sulfates, sulfides, bromides, iodides, chlorides, and fluorides. The fluids contain from 1 to 2 wt. % of the anhydrous salt or 1.5 to 6 wt. % of the salt with water of crystallization, and the fluids may also contain from 0.2 to 30 wt. % of corrosion inhibitors such as alkali nitrite or triethanolamine. This disclosure relates only to the cooling function of the fluid, to increase tool life, and does not mention the corrosion inhibiting properties of any of the disclosed fluid component other than the nitrite and triethanolamine, both of which inhibitors are known to the art. Indeed, many of the anions in the disclosed list, such as sulfates, sulfides and halides, are known to be highly corrosive. Thus, their inclusion in a metalworking fluid is unlikely to inhibit corrosion. This disclosure therefore does not suggest any corrosion inhibition by any of the disclosed anions (other than the nitrite) in a water-based metalworking fluid.